Where to start...
First and foremost, what the hell is up with the lag? If more then 10 enemies are on the screen at one time, or I decide to use rage mode or a spell, it lags so much my mouse pointer still tries to click where it was then said action was originally taken for a few seconds. Also, having a way to tell what quality you're at would be a good thing, too.
Second, why do you feel it necessary to not explain anything in the game? You never mention Rage mode needs kills to activate, and barely mention that spells do on the skill level up screen.
Ninth, why is there no mouse over or separate screen or any bloody way to check what your current upgrades do, instead of just what the next one is. This is especially annoying when you max an upgrade out and you don't get any indication at all as to what it does.
Seventeenth, pause feature stopped working on me after the first time I used it, might want to look into that.
Potato, what is a 100% completed level exactly? Another thing that's never indicated on how to even remotely get.
Purple, after beating level 20 and continuing to level 21 and beyond, there's an enemy unit that rapidly swaps between the sprites of every humanoid unit coming down from the top left now and again that dies in one hit from a max upgraded strength hit, is that supposed to be there?
Cupid, it's really bloody annoying that there's no way to continue a game at all, especially since you use wording on some screens that made it seem like you were going to implement a save system, but never did ("New Game" on the title screen and the wording of the pause screen under the quit option)
Plasma, speaking of screens, the layout and functionality of every screen in the game, save the achievement screens, feels rushed and unpolished. It seems weird to be that you would have the skill level up screen before the general upgrades, and to get to it again you have to go "back", instead of following a more intuitive phrase like "skill upgrades" or similar.
And that's pretty much all I can think to bitch about right now.
LT;DR: Game needs lots of work